380 



ALTERNATING CURRENTS 



wire secondary mains, which supply the local lighting loads. 

 The power consumers are usually connected to the secondaries 

 of V-connected or delta-connected transformers located at the 

 consumer's premises. In order that the secondary mains may 

 not be too large, 440 and 550 volts are generally used for the 

 power loads. (Also see Vol. I, page 380, Fig. 341.) 



In the sub-station, other power-transforming apparatus may be 

 installed, such as constant-current transformers; motor-generator 

 sets or synchronous converters, for obtaining direct current, etc. 

 158. Transmission Line Reactance ; Single-phase. In making 

 line calculations for the transmission of direct-current power, the 

 resistance alone needs to be considered. In making similar calcu- 

 lations for alternating-current lines, it is necessary to take into 

 consideration not only the line resistance, but the line reactance as 

 well. In cables and in overhead lines operating at high voltage, it 

 is also necessary to consider the capacitance between conductors. 



Figure 344 shows the cross-section of a two-conductor, single- 

 phase line. As the current at any instant flows in opposite direc- 

 tions in the two conductors, the circular paths of the magnetic 

 lines set up about one conductor must always go in a direction 

 opposite to that for the other conductor. That is, when one 



magnetic field is acting in a 

 clockwise direction, the other 

 must be acting in a counter- 

 clockwise direction. This 

 causes the two fields to act in 

 conjunction in the area be- 

 tween the two conductors, 



FIG. 344. Magnetic field between the as shown in Fig. 344. Thus, 

 two conductors of a single-phase line. twQ para H e l w i res f orm a rec _ 



tangular loop of one turn, through which flux is set up by the 

 current in the wires. This flux links the loop and the circuit 

 has inductance, therefore. It might appear that this inductance 

 would be negligible, because the loop has but one turn and the 

 flux path is entirely in air. It must be remembered, however, 

 that the cross-sectional area of the flux path is large, usually being 

 from 1 to 20 ft. wide and several miles long. Although the 

 flux density is small, the total flux linking the loop is usually 

 considerable. 



